1. Field of the Invention
The present invention relates to a device and a method for modulating the light from a light source via a plurality of light modulation elements. In particular, the present invention relates to a light modulation device and optical display device, and light modulation method and image display method, which are appropriate for implementing widening of the luminance dynamic range and number of gradations, for enhancing the luminance of the displayed images and their picture quality, and for making the entire device more compact.
Priority is claimed on Japanese Patent Application No. 2004-35091 filed 12 Feb. 2004, Japanese Patent Application No. 2004-48015 filed 24 Feb. 2004, and Japanese Patent Application No. 2004-54185 filed 27 Feb. 2004, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, the improvement in the picture quality of optical display devices such as LCDs (Liquid Crystal Displays), EL (electro-luminescence) displays, plasma displays, CRTs (Cathode Ray Tubes), projectors and the like has been remarkable. In terms of resolution and color range, performance which is almost as good as the visual characteristics of the human eye is now being implemented. However, when the luminance dynamic range is considered, its reproduction range only attains about from 1 to 102 [nit], and furthermore the number of gradations is generally 256 (expressed by 8 bits). On the other hand, the visual performance of the human eye is such that the dynamic range of luminance which can be perceived at one time is about from 10−2 to 104 [nit], and furthermore the luminance discrimination capability is about 0.2 [nit]. When these are converted into a number of gradations, it corresponds to around 4000 or more (i.e. as expressed by 12 bits). When a display image of a current optical display device is viewed via this type of human visual characteristic, the narrowness of the luminance dynamic range is conspicuous, and furthermore there is a perceived insufficiency with regard to the reality of the display image and its impact, since the gradations of the shadow portions and the highlight portions are insufficient.
Furthermore, in computer graphics (hereinafter abbreviated as “CG”) as used in films and games and so on, there is a mainstream trend to pursue reality of depiction by endowing the display data (hereinafter termed HDR (High Dynamic Range) display data) with a luminance dynamic range and a number of gradations close to those of the human visual performance. However there is the problem that, since the performance of the optical display device upon which this data is to be displayed is insufficient, it is not possible to provide a display which exhibits the CG contents with the power of expression that they originally had.
Furthermore, with next generation OS (Operating Systems), the adoption of 16-bit color space is anticipated, and the luminance dynamic range and the number of gradations will increase tremendously by comparison with the present 8-bit color space. Due to this, it is desirable to implement an optical display which can make the most of a 16-bit color space.
Among optical display devices, projection display devices such as liquid crystal projectors or so-called DLP projectors are devices which are capable of large screen display, and which are effective for reproducing the reality and impact of displayed images. In this field, in order to solve the above described problems, the following proposals have been made.
As a projection type display device of high dynamic range, for example, there is a technology which is disclosed in Japanese Patent Publication No. 2001-100689; this device comprises a light source, a first light modulation element which modulates the luminance of the light in all wavelength regions, and a second light modulation element which, for each of the three wavelength regions of the three primary colors R, G, and B among the wavelength region of the light, modulates the luminance in that wavelength region. A desired luminance distribution is created by modulating the light from the light source with the first light modulation element, and an optical image thereof is formed upon the picture element surface of the second light modulation element and is then color modulated thereby, and then the light which has thus been modulated in two stages is projected. Each of the picture elements (pixels) of the first light modulation element and the second light modulation element is controlled separately and individually based upon first control values and second control values which are determined from the HDR display data. For the light modulation elements, there are used transmission type light modulation elements which have a picture element (pixel) construction or a segmented construction of which the transmittivity ratios can be individually controlled, and which can control a two-dimensional transmittivity ratio distribution. A liquid crystal light valve may be offered as a representative example of such a device.
Now, the use of light modulation elements whose transmittivity ratio for dark display is about 0.2% and whose transmittivity ratio for bright display is about 60% is being considered. Considering such a light modulation element in isolation, its luminance dynamic range is thus 60/0.2=300. Since, with such a projection type display device according to the above described prior art, the luminance dynamic range corresponds to arranging two of such light modulation elements, each having a dynamic range of 300, optically in series with one another, it is accordingly possible to implement a luminance dynamic range of 300×300=90,000. Furthermore, with regard to the number of gradations as well, the same concept as the above is effective; it is possible to obtain a number of gradations which exceeds the 8-bit level by arranging light modulation elements which have 8-bit gradations optically in series.
Moreover, as a different type of projection type display device which implements a high luminance dynamic range, for example, there is a per se known type of projection type display device which is disclosed in Japanese Patent Publication No. 2001-174919.
This display device comprises a DMD (Digital Micromirror Device) in which a plurality of picture elements (pixels) are arranged in a linear configuration, an illumination section which radiates a ray bundle upon the DMD, a processing section which converts an input image signal into a drive signal for the DMD, an optical scanning section which scans the ray bundle which has been optically modulated with the DMD, and a projection lens which projects the ray bundle from the optical scanning section upon the surface of a screen; and the illumination section modulates the amount of light which it emits according to an input image signal.
FIG. 29 is a figure showing the picture element surface of the picture elements (pixels) of a transmission type liquid crystal light valve.
In such a transmission type liquid crystal light valve, a transistor for driving the picture element electrode and signal lines and the like are provided within the surface of each picture element (pixel). Accordingly, as shown in FIG. 29, an aperture section (a location through which light can be transmitted) is formed upon each picture element (pixel) in the shape of a window, and it is general for the opening ratio of this aperture section to be less than or equal to about 60%.
Accordingly, as explained in the '689 Japanese Patent Publication, in order to ensure the luminance of the displayed image, the optical image of the opening section of each picture element of the first light modulation element must be aligned so as to be formed accurately upon the opening section of the corresponding picture element (pixel) of the second light modulation element. In this structure, therefore, there is a requirement for this alignment to be performed at high accuracy.
FIG. 30 is a figure showing the structure of the optical path of the first light modulation element and the second light modulation element in the projection type display device disclosed in the '689 Japanese Patent Publication. It should be understood that, although other optical elements such as mirrors and so on are disposed upon the actual optical path, FIG. 30 has been drawn with these other optical elements eliminated for the convenience of the following explanation.
In the optical system of FIG. 30, a first light modulation element 130 for luminance modulation is disposed on the light source side of two fly-eye lenses 112a and 112b, and a second light modulation element 140 for color modulation is disposed on the opposite side to the light source of these two fly-eye lenses 112a and 112b. With this optical system, an optical image of the first light modulation element is formed upon the second light modulation element by the fly-eye lenses 112a and 112b and a condensing lens 112d. However, with the fly-eye lenses 112a and 112b and the condensing lens 112d, the image formation performance is low since they are optical elements which are used with the objective of making the luminance distribution uniform.
For this type of reason, in the structure described in the '689 Japanese Patent Publication, it is difficult to convey an optical image of the first light modulation element with good accuracy to the picture element surface of the second light modulation element. In this structure, there has been the problem that the luminance of the displayed image is deteriorated.
FIG. 31 is a figure showing the picture element surface of the picture elements of a reflective type light modulation element.
In contrast to a transmission type liquid crystal display element, with a reflective type light modulation element such as a DMD or the like, the light intercepting locations such as those of the signal lines and the drive transistors and so on are housed underneath the reflective picture element electrodes. It is possible, therefore, to provide the drive sections and the like which drive the mirrors on the rear surfaces of the mirrors. Accordingly, as shown in FIG. 31, for each of the picture elements (pixels), the opening ratio of its opening section (the location which reflects the light) generally becomes greater than or equal to 90%. Thus, if the first light modulation element and the second light modulation element are made as reflective type light modulation elements, it is possible to enhance the luminance of the displayed image.
In the '689 Japanese Patent Publication, the concept of being able to make the second light modulation element as a DMD is disclosed, but the following type of problem is still present.
With the optical system of FIG. 30, partial optical images of the first light modulation element 130 for each of the ranges which correspond to the constituent lenses which make up the fly-eye lens 112a which is close to the first light modulation element 130 are formed as superimposed upon the picture element surface of the second light modulation element 140. Due to this, in order to obtain the desired luminance distribution, it is necessary to form this luminance distribution for each of the ranges which correspond to the constituent lenses. However, the fly-eye lenses 112a and 112b are optical element which are utilized with the objective of making the luminance distribution uniform, and, for this objective, it is desirable for the number of their constituent lenses to be large. When this is done, inevitably, the size of each of the constituent lenses becomes small as compared to the size of the picture elements of the second light modulation element 140. In concrete terms, constituent lenses of ⅓ to ⅕ of the size are employed. Now, when it is considered to put the picture elements of the second light modulation element 140 and the picture elements of the first light modulation element 130 into one to one correspondence, the picture element density of the first light modulation element 130 necessarily becomes three to five times the picture element density of the second light modulation element 140. However, due to its very high definition, a current light modulation element (for example, a liquid crystal light valve) has a picture element density which is close to the upper limit of small-scale manufacturing technology, and, in consideration of this fact, it is difficult to implement a picture element density for the first light modulation element 130 which is three to five times greater. Accordingly, it inevitably becomes three to five times coarser than the fineness of the picture element density of the second light modulation element 140 in the accuracy of the luminance distribution which can be created by the first light modulation element 130. Moreover, an optical image of each of the constituent lenses of the fly-eye lens 112a is created upon the picture element surface of the second light modulation element 140 by the two lenses which are far from the first light modulation element 130, i.e. by the fly-eye lens 112b and the condensing lens 112d. Accordingly it is not possible to obtain sufficient aberration correction, and it is not possible to avoid a considerable amount of accompanying blurring.
Yet further, in the '689 Japanese Patent Publication, the transmission type light modulation element is simply replaced by a DMD. The following problems therefore arise.
As a projection type display device which employs a DMD, a projection type display device is proposed which utilizes an “off-axis optical system” of a type in which light is incident at a slanting angle upon the DMD, and in which light is caused to be emitted in a direction which is different from its direction of incidence.
However, in the structure described in the '689 Japanese Patent Publication, if the second light modulation element is arranged so as to constitute an off-axis optical system, the optical image of the first light modulation element is formed at a slanting angle with respect to the picture element surface of the second light modulation element. To this end, a distortion of trapezoidal shape is created at the picture element surface of the second light modulation element. Due to this, if two stage modulation is performed in which a desired light intensity distribution is created at the first light modulation element, and an optical image thereof is formed upon the picture element surface of the second light modulation element, then, distortion of a trapezoidal shape is created in the image which is formed by the first light modulation element. It is not possible, therefore, to perform such two stage modulation accurately. Accordingly, it becomes impossible to reproduce an image of the reality and the impact which the HDR display data originally possessed.
On the other hand, with the structure described in the '919 Japanese Patent Publication, a solid state laser or a semiconductor laser is used as the light source. There is the problem, therefore, that the device becomes rather large in size. Furthermore, if a semiconductor laser is utilized as the light source, there also is the problem that the luminance of the displayed image is deteriorated since its light output is rather low.